I don't think the seller is claiming the soft parts to be screen used. As far as I'm aware it's only the helmet, shoulder armour and shins.
The rest I believe is tour stuff.
Incidently AA was coerced into authenticating this. If anyone had spent as much time as I, AA and Jez did speaking on the phone you would realise that he is a very intense person and believed that AA was an expert on Vader as he work on ANH. Andrew did try and tell him that he wasn't and asked me to help him authenticate it, not that I'm quallified to do so either but I do know more than Andrew. Andrew never really wanted to do it but the seller practically insisted he did. That's why Andrew set up a meeting with Dave Prouse to try and satisfy the seller. He didn't know how or who else that would say it was an original.
We told the seller that there was doubt on it being screen used but it did look authentically a production piece and all Andrew could do is confirm that the screws and fixings etc were consistent with the era of the film. He doesn't pretend to know anything about Vader. It's the seller that has bigged up AA, not himself.
The seller has a long paper trail and is convinced it is what it is and didn't want to believe anything different.